This invention relates generally to etching and polishing compositions and particularly to a etch and polish composition for metal halides.
Metal halides, especially single crystal alkaline earth metal halides, have applications as semiconductor substrates for devices such as photovoltaic diodes, lasers, transistors, and charge coupled devices. For example, U.S. Pat. No. 4,080,723 discloses using single crystal barium fluoride as a substrate for group IV-VI semiconductor diodes to provide detectors for infrared radiation in thermal imaging systems. When employing metal halide salts, such as single crystal barium fluoride as a substrate for such semiconductor applications, it becomes necessary to etch, polish, or otherwise ablate a portion of the salt in order to insure a smooth substrate surface. Prior methods for obtaining a smooth substrate surface have included both mechanical and chemical techniques. U.S. Pat. No. 3,935,302 discloses a mechanical method for obtaining a smooth substrate surface which uses diamond impregnated blades. Recently the usefulness of chemical solutions to remove irregularities on crystal surfaces has been demonstrated by several investigators. Polishes have been developed for such materials as NaCl and KCl to produce high quality substrates for laser applications. U.S. Pat. No. 4,040,896 discloses a chemical polish for BaF.sub.2 and CaF.sub.2 which uses a mixed sulfuric acid and acetic acid solution for a predetermined immersion period not exceeding about 16 minutes per cycle for CaF.sub.2 but can be extended for several hours per cycle for BaF.sub.2. The preferred sulfuric to acetic acid parts ratio is about 4 to 9 for polishing the BaF.sub.2 crystal surface and about 3 parts to 2 parts for polishing the CaF.sub.2 crystal surface. U.S. Pat. No. 4,155,803 discloses the use of monovalent acids, including especially inorganic acids such as nitric acid, as well as halogen containing acids, as preferred hydrochloric acid, to chemically ablate the metal halide salt in aqueous solutions when exposed to low temperatures thereby producing a smooth surface.
Use of mechanical methods, however, to smooth the substrate surface can cause undesired chipping and cracking that impairs the usefulness of the device. Similarly, prior chemical methods have not obtained the required smoothness of the crystal surface which also limits the usefulness of the device.